Comparative study on the high-temperature tensile and creep properties of Alloy 617 base and weld metals

This paper presents a comparative investigation on the high-temperature tensile and creep properties of Alloy 617 base metal (BM) and weld metal (WM) fabricated by a gas tungsten arc weld process. The WM had higher yield strength and lower ultimate tensile strength than the BM does; however, its elongation was significantly lower than that of the BM. The creep curve of the BM and WM was somewhat different from that of typical heat-resistance steel, and did not show a textbook creep. The WM exhibited a longer creep rupture life, lower creep rate, and lower rupture ductility than the BM. However, as the creep rupture time reached approximately 36,800 h, the creep life of the WM was expected to be almost similar to that of the BM; and after 36,800 h, its creep life was expected to be worse than the BM. Loner creep tests is needed to investigate the long-term creep life of the WM. The creep failure mode of the BM and WM was obviously an intergranular cracking of the cavity formation and growth mechanisms, although it was more evident in the WM. The BM had a more ductile fracture surface than the WM.     

Reference stresses and temperatures for cylinders and spheres under internal pressure with a steady heat flow in the radial direction

The paper examines the creep behavior of thick cylinders and spheres subjected to internal pressure and a negative temperature gradient in the radial direction. It is found that at stationary state the rate of radial displacement of the vessel wall is simply proportional to the material creep behavior associated with a single stress and temperature. Such “reference stresses” and “reference temperatures” are defined for spheres and cylinders of varying wall thicknesses. These reference stresses and reference temperatures are valid for any creep problem where the material behavior may be characterized by a function of the form exp (γT)σ^m. The extension of these results to variable pressure and temperature loading cases is discussed.     

An experimental study of the ratchetting and creep of thick flanged tubes subjected to steady axial mechanical loading and transient thermal loading

The aims of the investigation were to obtain experimental data against which finite element predictions can be assessed and to see whether the lead alloy used was suitable as a “ratchetting” model material. Thermal ratchetting tests were performed on lead alloy flanged tube components. In some of the tests, dwell periods were allowed between successive thermal shocks. Electrical resistance strain gauges were used to measure the ratchet and creep strains in plain tube and stress concentration regions.It was found that both the plain tube and peak fillet ratchet strains increased with increasing mechanical and thermal load for short dwell periods. However, the ratio of the peak fillet to plain tube ratchet strains reduced with increasing mechanical and thermal load. Also, the ratio of the peak fillet to plain tube ratchet strains increased with increasing dwell period.The data obtained from the lead alloy model component tests were found to correlate with data from a number of different components made from various materials, indicating that the material may be useful as a “ratchetting” model material.     

An experimental and theoretical investigation into the creep properties of a simple structure of 316 stainless steel

The paper describes a set of creep experiments conducted on 316 austenitic stainless steel in a coupled pair of uniaxial creep testing machines which simulate the performance of a two bar structure subjected to constant mechanical and cyclic thermal loading in the temperature range of 550–600°C. The experimental results are compared with theoretical predictions using a non-interactive viscous creep/plasticity model and a model which includes recovery effects. In addition, an upper bound solution based upon the “rapid cycle” creep solution is calculated. The comparison shows that the recovery model is in close agreement with experiment and that the rapid cycle solution can provide a simple conservative estimate of the average creep rate.Several of the tests specimens exhibited strain instabilities during the initial cycles of temperature when sudden increases in inelastic strain of up to 0.2% occurred. As the material had not shown such effects in standard isothermal constant stress tests, the effects of simultaneous variation of stress and temperature which occurred in the two bar tests seems to have contributed to the occurrence of these strain instabilities.     

Creep characterization of type 316LN and HT-9 stainless steels by the K-R creep damage model

The Kachanov and Rabotnov (K-R) creep damage model was interpreted and applied to type 316LN and HT-9 stainless steels. Seven creep constants of the model,A, B. k, m, λ, γ, andq were determined for type 316LN stainless steel. In order to quantify a damage parameter, the cavity was interruptedly traced during creep for measuring cavity area to be reflected into the damage equation. For type 316LN stainless steel, λ=ε _R /ε* and λ _f =ε/ε _R were 3.1 and increased with creep strain. The creep curve with λ=3.1 depicted well the experimental data to the full lifetime and its damage curve showed a good agreement whenr=24. However for the HT-9 stainless steel, the values of A and A/ were different as λ=6.2 and λ _f =8.5, and their K-R creep curves did not agree with the experimental data. This mismatch in the HT-9 steel was due to the ductile fracture by softening of materials rather than the brittle fracture by cavity growth. The differences of the values in the above steels were attributed to creep ductilities at the secondary and the tertiary creep stages.     

Effect of ductile fracture criteria on chevron crack formation and evolution in drawing

The purpose of this study is to clarify the mechanics of ductile fracture in bulk metal forming processes by finite-element analysis and experiments. The author has developed a computer program, based on a conventional computer program involving the finite-element method, by which the behavior of crack propagation after ductile fracture can be analyzed. The phenomenon in which a ‘chevron crack’ appears periodically in the axial direction during drawing has been simulated using the developed computer program. Special attention is focused on the effect of various kinds of ductile fracture criteria on chevron crack formation and evolution during drawing. Results obtained are summarized as follows. First, the analytical results obtained using Gurson's fracture criterion and using Oyane's fracture criterion agree well with the experimental result. Second, the analytical results using Cockcroft and Latham's fracture criterion and using Brozzo et al.'s fracture criterion agree somewhat with the experimental result. Finally, the analytical result obtained using Freudenthal's fracture criterion does not agree with the experimental result.     

Damage evolution in creep bulging of thin sheet metal

The creeping motion of thin sheet metal, damaged by artificial cavities is observed in bulging tests and simulated ‘semi’-analytically. The sheet metal satisfies Norton’s Law for secondary creep and is subjected to a bi-directional stretch. The stretch is produced by creep bulging through elliptical dies with the virtue of sustaining nearly uniform background stress ratio for each aspect ratio of the die axes. In order to reach large deformations with significant shape evolution of the cavities, the tests were conducted at superplastic conditions. The sheet is double layered (only one layer is cavitated) made of Tin–Lead (50–50 Pb–Sn). The measured damage growth is compared to an approximate simulation. The simulation of the damage evolution, throughout its time history, makes repeated use of a so-called “Green-function solution” for the motion of a single isolated cavity in an infinite viscoplastic continuum. The solution is modified from Muskhelishvili’s elastic solution by replacing the elastic shear modulus by a “viscous-like” variable (“plastic shear modulus”) which depends (non-linearly) on the evolved average strain-rate. Similarly, the stresses in the ligaments between cavities were averaged to approximate the local stress concentrations. Due emphasis is given to the rotation of each elliptical cavity, beside its expansion (contraction) and elongation.     

Crack propagation in non-proportionally scaled elastic structures

Scaling laws for load, stress, displacement and crack velocity are given for crack propagation in non-proportionally scaled elastic structures where the scaling factors for height, width, thickness and crack length can all be different. Problems are addressed where only one or two of the scaling factors are important, in contrast to geometrically similar scaling where all factors are equal. Examples discussed include pipelines and plate-like structures of ships or bridges. The conditions under which strength of materials design criteria, based on yielding, prove inadequate for such large structures are investigated. In addition, the validity of determining “brittle” fracture toughness by “equivalent energy” scaling procedures from small laboratory specimens is discussed.     

Transition temperature and fracture mode of as‐castand austempered ductile iron

The ductile to brittle transition temperature is a very important criterion that is used for selection of materials in some applications, especially in low‐temperature conditions. For that reason, in this paper transition temperature of as‐cast and austempered copper and copper–nickel alloyed ductile iron (DI) in the temperature interval from ?196 to +150°C have been investigated. The microstructures of DIs and ADIs were examined by light microscope, whereas the fractured surfaces were observed by scanning electron microscope. The ADI materials have higher impact energies compared with DIs in an as?cast condition. In addition, the transition curves for ADIs are shifted towards lower temperatures. The fracture mode of Dls is influenced by a dominantly pearlitic matrix, exhibiting mostly brittle fracture through all temperatures of testing. By contrast, with decrease of temperature, the fracture mode for ADI materials changes gradually from fully ductile to fully brittle.     

一种用于热端部件高温蠕变分析的统一型粘塑性本构模型──模型分析之一

为了描述韧性材料的率相关力学行为，提出了一个具有“过应力”特征的统一型粘塑性本构模型。通过模型分析的方法，将该模型与典型的Ｂｏｄｎｅｒ－ｐａｒｔｏｍ统一型粘塑性本构模型进行了对比分析。重点考察了该模型对工程材料三种基本变形过程的描述能力：（１）瞬时塑性。（２）循环塑性。（３）蠕变变形（包括应力松驰、蠕变恢复）。     

An investigation into parallel and cross grinding of BK7 glass

Conventional grinding of BK7 glass will normally result in brittle fracture at the surface, generating severe sub-surface damage and poor surface finish. The precision grinding of BK7 glass in parallel and cross grinding modes has been investigated. Grinding process, maximum chip thickness, ductile/brittle regime, surface roughness and sub-surface damage have been addressed. Special attention has been given to the condition for generating a ductile mode response on the ground surface. A polishing–etching method has been used to obtain the depth of sub-surface damage. Experiments reveal that the level of surface roughness and depth of sub-surface damage vary differently for different grinding modes. This study gives an indication of the strategy to follow to achieve high quality ground surfaces on brittle materials.     

The determination of ductile creep rupture tim for orthotropic sheets and cylindrical shells

Equations describing the deformation of orthotropic materials for which a logarithmic creep rate in the form of a power function of the stresses are obtained. These equations are then used for the determination of the ductile creep rupture time of orthotropic sheets and cylindrical shells in biaxial tension. They can also be used in superplastic orthotropic metal forming calculations.     

Molecular dynamics (MD) simulation of uniaxial tension of some single-crystal cubic metals at nanolevel

Molecular Dynamics (MD) simulations of uniaxial tension at nanolevel have been carried out at a constant rate of loading (500 ms⁻¹) on some single-crystal cubic metals, both FCC (Al, Cu, and Ni) and BCC (Fe, Cr, and W) to investigate the nature of deformation and fracture. Failure of the workmaterials due to void formation, their coalescence into nanocracks, and subsequent fracture or separation were observed similar to their behavior at macroscale. The engineering stress–strain diagrams obtained by the MD simulations of the tensile specimens of various materials show a rapid increase in stress up to a maximum followed by a gradual drop to zero when the specimen fails by ductile fracture. The radius of the neck is found to increase with an increase in the deformation of the specimen and to decrease as the ductility of the material decreases. In this investigation, the strain to fracture is observed to be lower with the BCC materials than FCC materials. In the case of BCC crystals, no distinct linear trend in the engineering stress–strain characteristics is observed. Instead, rapid fluctuations in the force values were observed. If the drop in the force curves can be attributed to the rearrangement of atoms to a new or modified crystalline structure, it appears that BCC materials undergo a significant change in their structure and subsequent realignment relative to the FCC materials, as previously reported in the literature. While good correlation is found between the D- and α-parameters of the Morse potential with the ultimate strength and the strain to failure for the FCC metals, no such correlation is found for the BCC metals. From this, it appears that Morse potentials may not represent the deformation behavior of BCC metals as accurately as FCC metals and alternate potentials may need to be considered.     

Specific grinding energy as an in-process control variable for ductile-regime grinding

This article addresses the problem of monitoring the material removal regime (ductile versus brittle) that occurs during the grinding of brittle materials. Often a ductile grinding regime is desired, but currently there is no way to measure the grinding ductility “in process.” A model is developed to describe the dependence of the specific grinding energy on the material removal regime. It is found that the specific grinding energy will remain relatively constant for ductile-regime grinding but will decrease in a power-law relationship with an increasing material removal rate for brittle-regime grinding. Experimental confirmation of the proposed model is presented. The potential for using measurements of specific grinding energy to control the grinding ductility is established, and the benefits of such a closed-loop feedback system in ductile-regime grinding are explained.     

Non-linear one-dimensional continuum damage theory

The background of Non-Linear Continuum Damage Mechanics is presented for the case of one-dimensional tensile stresses. A brief critical analysis of the existing methods of non-linear damage models construction is given. A new approach to the damage theory development based on the “separability” principle is suggested. The condition of non-linearity and the criterion of long-term failure are formulated. The non-linear damage constitutive equations for static and cyclic loading, allowing one to take into account the loading history, are proposed on this basis. Within the framework of the approach suggested the problems of total and residual lifetime calculation under additional loads and partial unloads are solved. The predictions are compared with experimental data obtained using some particular heat-resistant materials.     

Stress and failure probability analysis for a transversely isotropic, brittle, elastic cylinder subjected to internal pressure and axisymmetric thermal loading

Procedures are developed for the determination of the stresses in and thence the probability of failure of a transversely isotropic cylinder made of a brittle material and loaded by an internal pressure and an axisymmetric radial temperature gradient. As examples of the application of the procedures a cylinder is analysed first with isotropic material properties, then with various degrees of anisotropy including both the “fibrous” and “laminar” types. The treatment is non-dimensional; results are presented graphically in the form of failure probability “contours”. For the dimensions and materials considered it is shown that the probability of failure is affected only slightly by the fibrous form of anisotropy but markedly by the laminar form when the thermal loading predominates.     

Comparison of experimental and theoretical residual stresses in welds: The issue of gauge volume

Welding residual stresses have an effect on many aspects of the integrity of structures but are normally one of the largest unknown stresses. Residual stresses are difficult to measure and to estimate theoretically but are often significant when compared with the service stresses on which they superimpose. High tensile residual stresses can lead to loss of performance in corrosion, fatigue and fracture.In this research, measurement of residual stresses by the neutron diffraction technique is compared to an analysis of a sample geometry by theoretical finite-element procedures. The results indicate good qualitative agreement. One of the key issues in this comparison relates to what is termed “gauge volume” in the measurement technologies and what might be described as a “calculation volume” in theoretical approaches.     

Fracture limit prediction using ductile fracture criteria for forming of an automotive aluminum sheet

Forming limit curves at neck and at fracture have been experimentally determined, and surfaces of fractured dome specimens have been observed optically and in the SEM, for an automotive AA6111-T4 sheet material. Various continuum ductile fracture criteria from the literature along with the assumptions of power law hardening, Hill’s quadratic yield criterion, and proportionality of stress and strain paths have been utilized for prediction of forming limit curve at fracture and compared with the experimental curve to assess the applicability of the different fracture criteria. The maximum shear stress criterion by Tresca predicts reasonably well the fracture limits of AA6111-T4 sheet material for a range of strain ratios, and is consistent with the microstructural observations. The criterion can be used to predict fracture limit curves from uniaxial tensile data and plane strain limit at fracture. A methodology for incorporating such a ductile fracture criterion into FE simulations of sheet stampings for prediction of fracture is discussed.     

Modelling of fretting fatigue in a fracture-mechanics framework

The use of fracture mechanics as an alternative to (Cauchy) stress-based fatigue criteria is illustrated in this paper, using the “crack analogue” concept to deal with crack initiation in a fracture mechanics framework. A very simple model, based entirely on independently derived parameters, is shown to be able to capture the qualitative effects of the normal and tangential loads of fretting-fatigue performance. The accuracy of the total life predictions is also satisfactory. Examples of how to account for residual stresses and size effect with such a model are discussed.